1. Field of the Invention
The present invention relates generally to optical devices such as lasers, and fiber optic data transmission systems employing the same. Particularly, the present invention is directed to a novel wavelength-locked loop servo-control circuit for optimizing performance of fiber optic transmission systems, particularly, by realizing efficient Polarization Mode Dispersion compensation over singlemode fiber in high-speed fiber optic data transmission systems.
2. Description of the Prior Art
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 mm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multi-layer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
In recent years, the efforts to develop very high data rate (10-40 Gbit/s) singlemode fiber optic transmission systems have had to deal with the fundamental limit imposed by Polarization Mode Dispersion (xe2x80x9cPMDxe2x80x9d). Like other types of pulse dispersion, PMD causes optical pulses to spread as they propagate through fibers, eventually causing intersymbol interference and bit errors. Effective compensation techniques have been proposed for other types of dispersion, such as modal and chromatic, leaving PMD as the fundamental limitation on the maximum data rate in a fiber optic communication system. Particularly, as illustrated in FIG. 1(a)-1(c), PMD arises because singlemode fibers transmit light in two modes, polarized horizontally 62 and vertically 63. As shown in FIG. 1(a), when the optical signal is launched, these polarization modes 62, 63 are initially in phase. The two modes mix freely in a circularly symmetric fiber, and even in a rectangular optical waveguide some degree of mode mixing will occur. In an ideal, homogeneous fiber, the two polarization states would not interact (FIG. 1(a)). If the interaction is very small (the two modes are degenerate) then there is little effect from the different polarizations. However, in all real fibers there are inhomogeneities, imperfections, micro and macrobends, and other environmental influences that affect the two polarization states differently. Variations in the fiber drawing process may also result in birefringence, sometimes equivalent to as much as a one wavelength shift between the two orthogonal polarization states of a signal at fiber optic wavelengths near 1550 nm, after transmission through only 10 meters of fiber. This results in a low level of birefringence (the two polarization states experience different effective indices of refraction). The birefringence can vary randomly along the length of the fiber, and interference between the two polarization states when they are not treated equally leads to PMD. For example, FIG. 11(b) illustrates how imperfections causing differences in the refractive indices of the two polarization modes, causes the speed of the two modes to vary through the fiber length, causing a delay 65. That is, for example, one polarization mode 63 lags behind the other mode 62. The signal dispersion caused by the difference in the arrival times of the two modes can limit system performance. Another phenomenon is mixing of the polarization modes which causes pulses to spread out in both polarizations such as shown in FIG. 1(c).
There is a related effect, polarization mode loss (PML), which describes the pulse spreading of PMD in terms of an effective optical power penalty on the fiber optic link budget. Polarization dependent loss can also arise when one polarization state is attenuated more than the other; this is essentially noise caused by the unintentional modulation of the light""s polarization. As data rates exceed 2.5 Gbit/s and approach 10 Gbit/s to 40 Gbit/s ranges, the pulse spreading caused by PMD can accumulate to levels that represent a significant fraction of the interval between bits. As shown in FIG. 1(c), because of the random nature of bending strain and environmental effects in an optical fiber, PMD accumulation causes the data pulses to spread out proportionally to the square root of fiber length (as opposed to chromatic dispersion, which is linearly proportional to fiber length). This statistical phenomena is called differential group delay. Each fiber has a characteristic PMD dispersion parameter, typically on the order of 0.05 to 1 picoseconds/{square root over (km)} (hereinafter ps/{square root over (km)}). It is understood that, PMD varies with time, optical wavelength, and operating conditions. Thus, for example, PMD may be much larger for outdoor cables suspended from telephone poles and subject to wind forces. PMD was typically not measured or specified for fibers installed more than a few years ago; due to more relaxed manufacturing controls at the time, such fibers are likely to have much higher levels of PMD. The PMD specifications noted above are average values; instantaneous PMD from a sudden change in cable properties can be much higher. In principle, all optical components exhibit some level of PMD, although fiber is the most important source because light travels furthest through fiber.
PMD impacts both analog and digital communication systems and, in the digital case, the end to end differential group delay caused by PMD should be no more than one-tenth the bit interval. As an example, a 10 Gbit/s system would require end to end PMD of no more than 10 ps. Thus, the fiber cable would have to meet specifications of less than 1 ps/{square root over (km)}; for a 100 km link or less than 0.1 ps/{square root over (km)} for a 10,000 km link. For a 40 Gbit/s system the total PMD is limited to 2.5 ps; a 10,000 km link would require a PMD of less than 0.025 ps/{square root over (km)}, which is lower than the best values currently available. Thus, current 40 Gbit/s transceiver designs are limited in their applications to installed legacy fiber. Optical amplifiers for long haul systems pass through accumulated PMD.
It is increasingly apparent that some form of PMD compensation is required in order to prevent these effects from limiting the distance and data rate of fiber optic links. However, it is the case that effective PMD compensation does not yet exist.
It would thus be highly desirable to provide an apparatus and method for realizing efficient PMD compensation over singlemode fiber in high-speed fiber optic data transmission systems.
It would thus be highly desirable to provide a system and method for automatically optimizing a fiber optic system by reducing the undesirable signal transmission effects caused by PMD, and particularly a system and method for overcoming the pulse spreading associated with PMD phenomena.
It is therefore an object of the present invention to provide an apparatus and method for realizing efficient PMD compensation over singlemode fiber in high-speed fiber optic data transmission systems.
It is another object of the present invention to provide a system and method for automatically optimizing a fiber optic system by reducing the undesirable effects caused by PMD, and particularly a system and method for overcoming the pulse spreading associated with PMD phenomena.
It is still another object of the present invention to provide a servo-control mechanism for automatically optimizing signal transmission in a fiber optic system which may be used in conjunction with standard PMD detector devices.
It is a therefore an object of the present invention to provide a servo-control loop for implementation with a PMD detector mechanism in a fiber optic transmission system that enables real-time compensation of input signal characteristics in order to provide optimum signal transmission with reduced PMD effects. It is a further object of the present invention to provide a servo-control loop for implementation with a PMD detector mechanism in a fiber optic transmission system that provides real-time PMD compensation by controlling input signal characteristics.
It is yet a further object of the present invention to provide a servo-control loop for implementation with a PMD detector mechanism in a fiber optic transmission system that provides real-time PMD compensation by controlling strain of the optical fiber.
Thus, according to the principles of the invention, there is provided a dispersion compensation system for an optical system comprising: an optical signal generator for providing an optical signal capable of being transmitted via a fiber optic link in an optical network, the optical signal characterized as having an operating center wavelength and the fiber optic length exhibiting means for causing polarization mode dispersion of the optical signal; a detector device for detecting an amount of polarization mode dispersion characteristic of the optical signal transmitted in the fiber link and generating an electric signal representing the detected amount; and, wavelength-locked loop servo-control circuit for automatically adjusting a peaked center wavelength of the optical signal in accordance with the detected amount of polarization mode dispersion to thereby minimize the polarization mode dispersion of the optical signal in the optical fiber link, the adjusted optical signal capable of being optimally transmitted over longer fiber distances with reduced dispersion effects in the optical network.
In a second embodiment, the wavelength-locked loop servo-control circuit is implemented for physically adjusting the optical fiber, e.g., by providing X-Y dimension strain control of the fiber, in accordance with a detected amount of polarization mode dispersion detected to thereby minimize the polarization mode dispersion of the optical signal in the optical fiber link.